Traveling wave tubes (TWT's) are utilized as power amplifiers at microwave frequencies for both radar and communications systems. A TWT is frequently built with a slow wave structure in the form of a helix, and with cathode and anode assemblies disposed at the ends of the helix for generating an electron beam which travels along the axis of the helix. Focussing of the beam is accomplished with an array of periodic permanent magnets which are of a toroidal shape for enclosing the helix and are positioned sequentially along the helical axis. The electron beam interacts with electromagnetic waves propagating along the helix to amplify the wave. One such wave, known as the fundamental wave, travels in the same direction as the electron beam, the forward direction, and is utilized for amplification of electric signals.
The helix also supports waves in the reverse direction, the backward waves. At a specific frequency which is greater than the normal amplification frequency band of the TWT, the backward wave can interact with the electron beam and extract energy therefrom, the extracted energy being transferred back toward the cathode assembly. Such transferrence of energy can introduce instabilities and oscillations to the TWT when the power of the backward wave is excessive. Various structures have been employed, such as resistive coatings and waveguide-type structures tuned to the backward wave frequency, for attenuating the backward wave so that the foregoing instabilities and oscillations do not occur.
A problem arises in that the structures which may be utilized in attenuating the backward wave are excessively large relative to the size of the helix to admit the mounting of the structures within the confines of the array of the permanent magnets utilized for the beam focussing.